1) Field of the Invention
The present invention relates to a medium conveyance apparatus for conveying a medium or media to make the medium pass through a processing section which processes (for example, transfers a visible image made with a developer) the medium under conveyance, and an image formation system for transferring a developer-made visible image onto a medium under conveyance to form the corresponding visible image on the same medium.
2) Description of the Related Art
Among image formation systems such as printers and copiers, there is an electrophotographic system in which a latent image is formed on a photosensitive unit and formed through a developer into a visible image which in turn, is transferred onto paper or a form (a sheet-like medium), one example of which has been disclosed in Japanese Unexamined Paten Publication No. 1-98529 (Japanese Paten Application No. 62-252524).
As described in the same publication, in this system. paper fed from a paper cassette is carried through a roller(s) to a transfer position, at which position a toner image formed on a surface of a photosensitive drum according to a well-known image formation procedure is transferred onto the paper fed. Following this, the paper with the transferred toner image is further conveyed to be discharged into a stacker after the toner image is fixed thereonto through the use of a fixing unit.
In such a system, the paper is conveyed in a state of being held between a preceding pair of rollers, and at this time, the front end portion of the same paper is made to be held between a succeeding pair of rollers prior to the rear end portion thereof being released from the preceding pair of rollers, so that the paper is held and conveyed by the succeeding pair of rollers after the separation from the preceding pair of rollers.
There is a problem which arises with this system, however, in that the paper traveling speed varies the moment the rear end portion of the paper is released from the preceding pair of rollers, and if this speed variation occurs in relation to paper under the transfer of a toner image at the transfer position, the toner image transferred onto the paper goes out of order. The state of this speed variation depends upon the environments including the roller temperatures.
In more detail, there is the time that the paper is conveyed in a state of being held by both the preceding and succeeding pairs of rollers as mentioned above, and the ideal is that the paper conveyance speeds due to the two sets of rollers are the same, and in practice, owing to the differences between various components such as rollers, motors and gears (put between the rotating drive shafts of the motors and the rotary shafts of the rollers for connection) on manufacturing, difficulty is experienced to always equalize the paper conveyance speeds by the two sets of rollers. In addition, if enhancing the manufacturing accuracy of the respective components in order to equalize the paper conveyance speeds, the manufacturing costs of the components increase to raise the system price. For this reason, in fact difficulty is also encountered to enhance the manufacturing accuracy of the respective components.
Therefore, taking the disorder or the like of an image transferred onto the paper into consideration, the conveyance speed due to the succeeding pair of rollers is set to be slightly higher than that by the preceding pair of rollers so that the paper is conveyed while being stretched between the preceding pair of rollers and the succeeding pair of rollers. This can prevent the occurrence of a paper waving phenomenon, and hence, in cases where the transfer position stands between the preceding pair of rollers and the succeeding pair of rollers, stable transfer is feasible at that transfer position. On the other hand, when the paper becomes free from the tension the moment that the rear end portion of the paper is released from being held between the preceding pair of rollers, the paper begins to flap and disrupts the conveyance of the rear end portion of the paper, thereby making it difficult to obtain a stable transfer of the toner image.